msem-instigator

When Undying Watcher enters the battlefield, germinate. (Put a +1/+1 counter on a creature or create a 1/1 green Saproling creature token.)
Sacrifice another creature: Undying Watcher gains indestructible until end of turn.

Enchant land
When Unexplored Regions enters the battlefield, draw a card.
Enchanted land is an artifact in addition to its other types and has “{T}: Add one mana of any color.”

Flying
When Unfettered Atrocity enters the battlefield, destroy target creature or planeswalker.

When Unite Against Evil enters the battlefield, return target creature card with mana value 3 or less from your graveyard to the battlefield.
Menagerie — Whenever you attack, if there are five or more creature types among creatures you control, put a +1/+1 counter on target attacking creature and untap it.

When United in Battle enters the battlefield, look at the top six cards of your library. You may reveal a Dwarf or Elf card from among them and put it into your hand. Put the rest on the bottom in a random order.
Whenever a Dwarf or Elf enters the battlefield under your control, put a +1/+1 counter on it.

When Unity Spheres enters the battlefield, create two 1/1 colorless Construct artifact creature tokens.
Tap two untapped creatures you control: Add two mana of any one color.

University Archive enters tapped.
When University Archive enters the battlefield, look at the top card of your library. You may put it into your graveyard.
{T}: Add {U} or {R}.

When Unmasker of Fate's Design enters the battlefield, look at target player's hand. Choose a nonland card from it and exile that card with a puzzle counter on it. As long as that card has a puzzle counter on it, that player may pay {2} and 1 life at any time. If they do, return the exiled card to their hand.

Flying
When Unraveller of Mysteries enters the battlefield, mill two cards.

This spell costs {2} less to cast if your starting deck contained at least forty cards more than the minimum deck size.
When Unrivaled Fireweaver enters the battlefield, you may exile a red instant or sorcery card you own from outside the game. Until the end of your next turn, you may play that card.

Glitch—Sacrifice a creature. (Remove one [restriction] for each time you pay the glitch cost.)
When Unshackled Revolutionary enters the battlefield, you may destroy target creature an opponent controls [with mana value 3 or less]. [You lose 3 life.]

When Unstable Orb enters the battlefield, draw a card.
{3}, {T}, Sacrifice Unstable Orb: It deals 2 damage to target creature.

Rhapsody {4}{U}{U} (Pay {4}{U}{U} to cast this as a 4/4, or as a sorcery to make it become a 4/4.)
When Unstable Prophet enters the battlefield, surveil X, where X is its power. (Look at the top X cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)

Whenever you cast a nongreen spell, creatures you control get +1/+1 until end of turn.
Wanderlust — When Untethered Disciple enters the battlefield, if you control four or more differently named lands, create two 1/1 Scout creature tokens that are all colors.

Untrekked Territory enters the battlefield tapped.
{T}: Add {B} or {G}.
When Untrekked Territory enters the battlefield, you may put a loyalty counter on a planeswalker you control.

When Unwavering Sentinel enters the battlefield, for each creature you control, create a white Aura enchantment token named Shield with “Enchant creature” and “If enchanted creature would be destroyed, instead remove all damage from it and sacrifice this Aura” attached to that creature.

Flying (This creature can't be blocked except by creatures with flying or reach.)
Menagerie — When Uplifting Companion enters the battlefield, if there are five or more creature types among creatures you control, up to two target creatures you control gain flying until end of turn.

When Urisk Fiddler enters the battlefield, draw a card.

When Urn of Heroes enters the battlefield, you may search your library for a legendary card, put that card into your graveyard, then shuffle.
{T}: Add {C}.

Haste
When Urud Stingswoop enters the battlefield, it gains flying until end of turn.

When Vagabones enters the battlefield, look at the top two cards of your library. Put any number of them into your graveyard and the rest on top of your library in any order.

When Vale Ruins enters the battlefield, you gain 1 life.
{T}: Sacrifice Vale Ruins: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Flying
When Valiant Instructor enters the battlefield, you return up to one target enchantment card from your graveyard to your hand.

When Valley's Majesty enters the battlefield, draw a card, then choose a card name.
Nonbasic lands you control with the chosen name have “{T}: Add two mana of any one color.”

Enchant creature
When Vampiric Rise enters the battlefield, fateseal 2. (To fateseal 2, look at the top two cards of an opponent's library, then put any number of them on the bottom of that player's library and the rest on top in any order.)
Enchanted creature gets +1/+0 and has flying.